CN113573837A - Lap laser spot welded joint, method for manufacturing same, and structural member for automobile body - Google Patents
Lap laser spot welded joint, method for manufacturing same, and structural member for automobile body Download PDFInfo
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- CN113573837A CN113573837A CN201980094063.9A CN201980094063A CN113573837A CN 113573837 A CN113573837 A CN 113573837A CN 201980094063 A CN201980094063 A CN 201980094063A CN 113573837 A CN113573837 A CN 113573837A
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K26/00—Working by laser beam, e.g. welding, cutting or boring
- B23K26/20—Bonding
- B23K26/21—Bonding by welding
- B23K26/22—Spot welding
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K26/00—Working by laser beam, e.g. welding, cutting or boring
- B23K26/20—Bonding
- B23K26/21—Bonding by welding
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K35/00—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting
- B23K35/22—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting characterised by the composition or nature of the material
- B23K35/24—Selection of soldering or welding materials proper
- B23K35/30—Selection of soldering or welding materials proper with the principal constituent melting at less than 1550 degrees C
- B23K35/3053—Fe as the principal constituent
- B23K35/3073—Fe as the principal constituent with Mn as next major constituent
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K2101/00—Articles made by soldering, welding or cutting
- B23K2101/006—Vehicles
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K2101/00—Articles made by soldering, welding or cutting
- B23K2101/18—Sheet panels
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- Optics & Photonics (AREA)
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- Laser Beam Processing (AREA)
Abstract
A lap laser spot welded joint, a method of manufacturing the same, and a structural member for an automobile body having the same are provided, in which, when a plurality of steel sheets are vertically overlapped and a laser beam is intermittently irradiated onto one side surface of the overlapped steel sheets to form a welded part including joint portions arranged continuously in a row, the shape of the joint portions is made into a large oval joint portion by at least spirally scanning the laser beam from the outside toward the inside of the oval shape while drawing an elongated circle combining a semicircle and a straight line, and welding conditions are controlled so that the minor axis width D of the oval joint portion becomes large1Major axis width D2、D2Relative to the above D1Ratio (D)2/D1) And the minimum thickness u of the final solidified part satisfies a predetermined condition so as not to be present in the joint partCracks were generated in the welded terminal portion, and the peel strength was also excellent.
Description
Technical Field
The present invention relates to a lap laser spot welded joint, a method for manufacturing the same, and a structural member for an automobile body having the lap laser spot welded joint.
Background
Resistance spot welding is generally used for welding structural members (strength members) of an automobile body having a flange portion. However, resistance spot welding has various problems such as a long time required for welding, an inability to reduce a welding pitch due to a decrease in heat generation amount due to shunting, and a need for a certain amount of space for installing a welding torch. In order to solve these problems, in recent years, a technique of lap laser spot welding has been studied and widely used instead of conventional resistance spot welding. Here, the lap laser spot welding is a welding method in which a laser beam is irradiated to one surface of a plurality of steel sheets stacked one on another to melt and join the steel sheets.
Conventionally, in a lap laser spot welded joint, a laser beam is intermittently irradiated onto the surfaces of a plurality of superposed steel sheets, the steel sheets at the positions irradiated with the laser beam are melted and solidified, and a welded portion in which linear or circular joint portions are continuously arranged in a row is formed, thereby joining the plurality of steel sheets. However, when the joint portion is linear, there is a problem that cracks are likely to occur in a final solidification portion located on the welding end portion side of the joint portion in the lap laser beam welding. In addition, when the joint portion has a circular shape, there is a problem that cracks are likely to occur in a final solidified portion located at the center of the joint portion. When a crack is generated, the crack propagates over the entire length of the joint portion, and therefore not only the static strength such as the shear strength and the peel strength of the welded joint portion is reduced, but also the fatigue strength is significantly reduced. In recent years, high tensile steel sheets have been often used for automobile body parts, particularly structural parts (strength parts) as frame parts, in order to improve the strength and rigidity of the automobile body, and a reduction in the static strength and fatigue strength of a welded joint due to cracks generated at a joint portion has become a serious problem.
Therefore, various methods have been studied to prevent cracks from occurring at the welding end portions of the joint portions when laser beam welding is performed on the overlapped steel sheets. For example, patent document 1 discloses that welding cracks are prevented by projecting a lower steel plate to be lap-welded and setting a welding start position to a position apart from a flange end portion. Patent document 2 discloses a technique of preventing weld cracking by irradiating an end portion of the faying surface with laser light from an oblique direction. Patent documents 3 and 4 disclose techniques for preventing weld cracks by reheating or welding the welded portion and the periphery of the welded portion. Patent document 5 discloses a technique for preventing weld cracking by welding a lap surface in an elliptical shape. Patent document 6 discloses a technique for preventing weld cracking by optimizing the steel sheet composition and optimizing the ratio of the bead width to the bead thickness.
Documents of the prior art
Patent document
Patent document 1: japanese patent laid-open No. 2007-229740
Patent document 2: japanese laid-open patent publication No. 2008-296236
Patent document 3: japanese laid-open patent publication No. 2012-240083
Patent document 4: japanese laid-open patent publication No. 2012-240086
Patent document 5: japanese patent laid-open publication No. 2017-113781
Patent document 6: japanese patent laid-open publication No. 2018-001197
Disclosure of Invention
Problems to be solved by the invention
However, in the method described in patent document 1, since the lower steel sheet to be lap-welded is protruded, the protruded portion becomes an extra portion, and there is a problem that the design of parts is limited. In addition, in the method described in patent document 2, since the laser beam is irradiated from an oblique direction, when the overlapped plates are spaced apart from each other by a gap, a melted portion cannot be formed satisfactorily on the faying surface, and the penetration becomes insufficient, which causes a problem that it is difficult to secure sufficient strength. In addition, in the methods described in patent documents 3 and 4, since it is necessary to reheat or weld the periphery of the welded portion or the welded portion, there is a problem that the welding time becomes long. In addition, the method described in patent document 5 has a problem that sufficient welding strength cannot be secured because the shape of the welded portion is limited to a circular shape or an elliptical shape close to a circular shape. Further, in the method described in patent document 6, since stress tends to concentrate at the welding terminal end, there is a problem that it is not possible to prevent cracks from occurring at the welding terminal end of the linear joining portion having a short length.
The present invention has been made in view of the above problems of the prior art, and an object of the present invention is to provide a lap-welded joint having a welded portion formed by intermittently irradiating a laser beam so as to form a joint portion (spot) in a row, a lap-laser spot-welded joint in which cracks are not generated in a finally solidified portion of the joint portion and peeling strength of the welded portion is excellent, a method for manufacturing the lap-laser spot-welded joint, and an automobile body structural member having the lap-laser spot-welded joint.
Means for solving the problems
In order to solve the above problems, the inventors of the present application have made intensive studies with attention paid to the shape and size of each joint (nugget) constituting a welded portion formed by laser welding. As a result, it was found that in order to prevent cracking of the final solidified portion of the joint portion, it is effective to form the joint portion into an oblong shape larger than a conventional linear shape, circular shape, or elliptical shape, and to control various dimensions of the oblong joint portion within appropriate ranges, and the present invention was finally developed.
The present invention based on the above-described findings is a lap laser spot welding joint having a welded portion formed by overlapping a plurality of steel plates, wherein a total gap G between the steel plates constituting the welded portion is in a range of 0 to 15% of a total thickness T of the steel plates constituting the welded portion, the welded portion includes oblong joined portions arranged intermittently, and the oblong joined portions satisfy all of the following expressions (1) to (5):
1.0≤T≤6.0 (1)，
2.0≤D1≤8.0 (2)，
6.0≤D2≤15.0 (3)，
1.1≤D2/D1≤5.0 (4)，
0.6≤u/T≤1.0 (5)，
wherein T is a total plate thickness (mm) of the steel plates constituting the welded portion,
D1is the minor axis width (mm) of the oblong joint,
D2the long axis width (mm) of the oblong joint,
u is the minimum thickness (mm) of the final set portion of the oblong joint.
The lap laser spot welded joint of the present invention is characterized in that at least 1 of the steel sheets has a composition containing C: 0.07 to 0.4 mass%, Si: 0.2 to 3.5 mass%, Mn: 1.8-5.5 mass%, P + S: 0.03 mass% or less, Al: 0.08% by mass or less and N: 0.010 mass% or less, and the balance of Fe and inevitable impurities.
In addition, the lap laser spot welded joint of the present invention is characterized in that the steel sheet contains, in addition to the above composition, at least 1 component from the following groups a and B:
group A is selected from Ti: 0.0005 to 0.01 mass% and Nb: 0.005-0.050% by mass of 1 or 2 kinds selected from;
group B is selected from Cr: 1.0 mass% or less, Mo: 0.50 mass% or less and B: 0.10% by mass or less of 1 or 2 or more selected from the group.
In the lap laser spot welded joint according to the present invention, at least 1 of the steel sheets is a high-tensile steel sheet having a tensile strength of 980MPa or more.
The present invention also provides a method of manufacturing a lap laser spot welded joint, in which a plurality of steel sheets are vertically stacked, a laser beam is intermittently irradiated onto one side surface of the stacked steel sheets, and a welded portion including oblong joints arranged in a row is formed, thereby manufacturing a lap laser spot welded joint, the method being characterized in that a total gap G between the steel sheets constituting the welded portion is set to be in a range of 0 to 15% of a total thickness T of the steel sheets constituting the welded portion, the oblong joints are spirally scanned from an outer side to an inner side of the oblong while the laser beam rotates so as to draw an elongated circle in which semicircles and straight lines are combined, and at least 1 of a laser power, a focal position, a welding speed, a spin radius, a movement amount per 1-time spin, and a beam diameter is controlled, such that the oblong joint portion satisfies all of the following formulas (1) to (5):
1.0≤T≤6.0 (1)，
2.0≤D1≤8.0 (2)，
6.0≤D2≤15.0 (3)，
1.1≤D2/D1≤5.0 (4)，
0.6≤u/T≤1.0 (5)，
wherein T is a total plate thickness (mm) of the steel plates constituting the welded portion,
D1is the minor axis width (mm) of the oblong joint,
D2the long axis width (mm) of the oblong joint,
u is the minimum thickness (mm) of the final set portion of the oblong joint.
The present invention is an automotive body structural member having any one of the lap laser spot welded joints described above.
ADVANTAGEOUS EFFECTS OF INVENTION
According to the present invention, the joint portion of the welded portion constituting the lap laser spot welded joint obtained by laser beam welding the plurality of steel sheets stacked one on another is formed into the oval joint portion larger than the conventional one, whereby the lap laser spot welded joint can be manufactured which can reliably suppress the occurrence of cracks in the final solidified portion and which has excellent peel strength of the welded portion. In addition, since the lap laser spot welded joint of the present invention can form an oblong joint portion having a wide range of major-minor axis ratio, it is possible to improve the degree of freedom in designing components and to develop components that are lighter in weight, higher in rigidity, and higher in strength. Therefore, the lap laser spot welded joint of the present invention can be applied to a structural member (strength member) that becomes a framework of an automobile body.
Drawings
Fig. 1 is a perspective view showing an example of a conventional lap laser spot welded joint.
Fig. 2 is a schematic view illustrating a welded portion of a conventional lap laser spot welded joint, where (a) is a plan view and (b) is a cross-sectional view taken along line a-a of (a).
Fig. 3 is a schematic view illustrating a welded portion of the lap laser spot welded joint according to the present invention, wherein (a) is a plan view and (B) is a cross-sectional view taken along line B-B of (a).
Fig. 4 is a diagram for explaining a welding method used for manufacturing a welded joint according to the present invention.
Fig. 5 is a diagram illustrating an example of a scanning trajectory of a laser beam for obtaining the oval joint according to the present invention.
Fig. 6 is a view illustrating a welding position of the lap laser spot welded joint of the present invention, wherein (a) is a plan view, and (b) is a C-C sectional view of (a).
Fig. 7 is a perspective view illustrating a peel test piece having a lap laser spot weld joint used in an example of the present invention.
Detailed Description
The lap laser spot welded joint, the method for producing the lap laser spot welded joint, and the structural member for an automobile body having the lap laser spot welded joint according to the present invention will be described below.
< lap laser spot welded joint >
Fig. 1 is a perspective view showing an example of a conventional lap laser spot welded joint. The lap laser spot welded joint 1 has a structure in which at least 2 steel plates are stacked, and in the example shown in fig. 1, 2 steel plates, each having a vertical wall portion 2a and a flange portion 2b extending outward from the front end of the vertical wall portion 2a, each having a substantially hat-shaped cross-sectional shape, and a flat panel-shaped steel plate 3, are stacked so that the flange portion 2b faces the steel plate 3 to form a joint surface, and a laser beam is irradiated from above the flange portion 2b onto the surface of the flange portion 2b to form a molten portion (molten metal portion) penetrating at least the steel plate 2 and solidify to form a joint portion (spot), whereby welding is performed. Although a Heat Affected Zone (HAZ) is present around the fusion zone, the joint zone of the present invention is simply a fusion zone other than the heat affected zone.
The welded portion of the lap laser spot welded joint 1 is formed by intermittently irradiating the surface of the flange portion 2b with a laser beam while moving a welding head as a laser beam source along the longitudinal direction (arrow direction in fig. 1) of the vertical wall portion 2 a. As a result, as shown in fig. 1, elliptical joint portions (welding spots) are continuously formed in a row on the joint surface of the steel sheet 2.
Fig. 2 is a schematic view showing a conventional welded portion formed in a flange portion 2b of the lap laser spot welded joint shown in fig. 1, where (a) is a plan view of a joint portion constituting the welded portion as viewed from above the flange portion 2b, and (b) is a cross-sectional view showing a cross-section a-a shown in (a).
In conventional laser beam welding, when a joint portion 14 having a long and narrow oval shape, i.e., a molten portion with a large minor axis-major axis ratio, is formed, a weld crack 5 starts to occur from a central portion 14a which is a final solidified portion, and therefore, it is necessary to reduce the minor axis-major axis ratio of the molten portion, which is a problem that it is difficult to improve the welding strength. This is because, as shown in fig. 2 (b), since a large amount of spatters are generated in the conventional laser beam welding, the center portion 14a which becomes the final solidified portion becomes too thin, and thus a tensile stress (a force in the direction of arrow σ a shown in fig. 2 (a)) from the outer peripheral portion of the molten portion toward the outside is concentrated. On the other hand, if the molten diameter of the joint portion is increased in order to improve the welding strength, there is another problem that burnthrough occurs.
The crack at the welding terminal end is caused in the final solidified portion of the joint portion so as to penetrate from the front surface to the back surface, and whether or not the crack is caused can be visually confirmed, but for more reliable determination, it is preferable that the final solidified portion of the joint portion after welding is cut in the width direction, and the cut surface is observed and determined by magnifying it to about 10 times using, for example, an optical microscope.
Therefore, the inventors of the present application have made extensive studies on measures for preventing the thickness of the central portion of the finally solidified portion which is the joined portion from decreasing in order to reduce the stress concentration occurring in the finally solidified portion of the joined portion in the laser beam welding.
As a result, as will be described later, the laser beam is caused to rotate while describing an elongated circle in which a semicircle and a straight line are combined, and the laser beam is scanned spirally from the outside toward the inside of the oval shape, thereby forming the oval-shaped bonded portion shown in fig. 3 (a), whereby occurrence of sputtering can be suppressed, and further, as shown in fig. 3 (b), the minimum thickness u of the central portion 4a of the bonded portion which becomes the final solidification portion can be increased. Therefore, by forming the oval joint portion, the tensile stress (arrow σ b shown in fig. 3 a) applied to the final solidified portion 4a of the joint portion from the outer peripheral portion of the melted portion toward the outside can be significantly reduced as compared with the conventional welding method, and therefore, the crack of the final solidified portion can be prevented. Here, the "oblong" in the present invention means a shape obtained by connecting 2 circles having equal radii with a common circumscribed line.
By adopting the oval joint portion larger than the conventional one, cracks in the finally solidified portion of the joint portion can be greatly reduced. However, as a result of further studies by the present inventors, in order to more reliably prevent cracking in the final solidified portion of the joint portion and to achieve sufficient strength in the peel strength of the welded portion, it is necessary to adopt the oblong joint portion such that the oblong joint portion satisfies 0 ≦ G/T ≦ 0.15 described later and all of the following equations (1) to (5):
1.0≤T≤6.0 (1)
2.0≤D1≤8.0 (2)
6.0≤D2≤15.0 (3)
1.1≤D2/D1≤5.0 (4)
0.6≤u/T≤1.0 (5)
wherein, T: total sheet thickness (mm) of steel sheets constituting the welded part
D1: minor axis width (mm) of oblong joint
D2: long axis width (mm) of oblong joint
u: minimum thickness (mm) of the final solidified portion of the oblong joint.
The following is a detailed description.
0≤G/T≤0.15
In the lap laser welded joint of the present invention, the ratio (G/T) of the total gap G between the steel plates constituting the welded portion to the total thickness T of the steel plates constituting the welded portion is required to be 0 to 0.15, that is, the ratio of the total gap G between the steel plates constituting the welded portion to the total thickness T of the steel plates constituting the welded portion is required to be in the range of 0 to 15%. The reason for this is that if the ratio of G to T exceeds 15%, the depth of the crater at the welding terminal end becomes deeper, and stress becomes more likely to concentrate. Preferably 0 to 10%.
1.0≤T≤6.0
In addition, the lap laser spot-welded joint needs to enable the total plate thickness T of a plurality of steel plates to be in a range of 1.0-6.0 mm. When the total sheet thickness is thinner than 1.0mm and thicker than 6.0mm, burn-through is likely to occur after laser beam irradiation, and it becomes difficult to perform lap welding. Therefore, the total plate thickness T of the steel plates constituting the welded joint of the present invention is in the range of 1.0 to 6.0 mm. Preferably 2.0 to 5.0 mm.
2.0≤D1≤8.0
In addition, the lap laser spot welded joint of the present invention requires the minor axis width D of the oblong joint portion constituting the welded portion1Is in the range of 2.0 to 8.0 mm. If D is1Burn through occurs above 8.0 mm. Preferably 6.0mm or less. On the other hand, from the viewpoint of securing sufficient bonding strength, D1Is more than 2.0 mm. Preferably 4.0mm or more.
6.0≤D2≤15.0
In addition, the lap laser spot welded joint of the present invention requires the major axis width D of the oval joint portion constituting the welded portion2Is in the range of 6.0 to 15.0 mm. If D is above2If the thickness is more than 15.0mm, weld cracks may occur. Preferably 13.0mm or less. On the other hand, D2The lower limit of (b) is not specified, and is 6.0mm from the viewpoint of securing sufficient bonding strength. Preferably 8.0mm or more.
1.1≤D2/D1≤5.0
In addition, the lap laser spot welded joint of the present invention requires the major axis width D of the oval joint portion constituting the welded portion2Width D relative to minor axis1Ratio (D)2/D1) Is in the range of 1.1 to 5.0. If the above ratio (D)2/D1) If the ratio is more than 5.0, weld cracking may occur. Preferably 4.0 or less. On the other hand, in the ratio (D)2/D1) If the ratio is less than 1.1 times, weld cracking is likely to occur. Preferably 1.5 or more.
0.6≤u/T≤1.0
In addition, the lap laser spot welded joint of the present invention is required to have a ratio (u/T) of a minimum thickness u of a melted portion generated in a central portion 4a of a final solidified portion of a joint portion to a total plate thickness T of steel plates constituting the joint portion in a range of 0.6 to 1.1. If the ratio (u/T) is less than 0.6, the tensile stress applied to the central portion 4a of the final solidified portion from the outer peripheral portion of the molten portion toward the outside increases, and it becomes impossible to prevent cracking. Preferably 0.7 or more. On the other hand, the thickness u of the molten portion, which is the central portion of the final solidified portion, is generally smaller than the total thickness T of the plurality of steel sheets by sputtering, and therefore the ratio (u/T) is 1.0 or less.
Next, a steel sheet constituting the lap laser spot welded joint of the present invention will be described.
In fig. 1 to 4, an example of a lap laser spot welded joint is shown in which 2 steel plates are overlapped, but a welded joint may be formed by overlapping 3 or more steel plates.
Thickness of steel plate
The total thickness of the steel sheets constituting the lap laser spot welded joint of the present invention has been described above, but the thickness of each steel sheet is not particularly limited, and may be in the range of 0.5 to 3.2mm, which is generally used as an outer panel or a structural member (strength member) of an automobile body. The plurality of steel sheets may have the same thickness or different thicknesses. For example, in the case of the lap laser spot welded joint 1 having the shape shown in fig. 1, the thickness t of the upper steel plate 2 may be set20.6 to 1.8mm, and the thickness t of the lower steel plate 33The thickness t of the upper steel plate 2 may be set to be in the range of 1.0 to 2.5mm2And the thickness t of the lower steel plate 33The thickness of the plate is 0.5 to 3.2 mm.
Composition of steel sheet
Further, the composition of the plurality of steel sheets constituting the lap laser spot welded joint of the present invention is not particularly limited, and preferably at least 1 steel sheet contains C: 0.07 to 0.4 mass%, Si: 0.2 to 3.5 mass%, Mn: 1.8-5.5 mass%, P + S: 0.03 mass% or less, Al: 0.08% by mass or less and N: 0.010 mass% or less, and the balance of Fe and inevitable impurities.
C: 0.07 to 0.4% by mass
C is an element contributing to the improvement of the strength of steel, and the precipitation strengthening and transformation strengthening effects can be obtained by containing 0.07 mass% or more of C. Further, by setting the C content to 0.4 mass% or less, coarse carbide precipitation can be avoided and desired strength and workability can be ensured. Therefore, the C content is preferably in the range of 0.07 to 0.4 mass%. More preferably 0.15 to 0.3 mass%.
Si: 0.2 to 3.5% by mass
Si is an element having excellent solid-solution strengthening ability, and the strength of steel can be improved by containing 0.2 mass% or more. In addition, by setting the Si content to 3.5 mass% or less, excessive solidification of the welding heat-affected zone can be suppressed, and deterioration of toughness and low-temperature cracking resistance of the welding heat-affected zone can be prevented. Therefore, the Si content is preferably in the range of 0.2 to 3.5 mass%. More preferably 1.0 to 2.5 mass%.
Mn: 1.8 to 5.5% by mass
Mn is an element effective for improving hardenability and suppressing precipitation of coarse carbides, and is preferably contained in an amount of 1.8 mass% or more. Further, by setting the Mn content to 5.5 mass% or less, the grain boundary embrittlement sensitivity can be suppressed from being improved, and deterioration in toughness and low-temperature crack resistance can be prevented. Therefore, the Mn content is preferably in the range of 1.8 to 5.5 mass%. More preferably 2.0 to 3.5 mass%.
P + S: 0.03 mass% or less
P and S are harmful elements that adversely affect the ductility and toughness of steel, and the total content of P and S is 0.03 mass% or less, whereby the reduction in ductility and toughness can be prevented and desired strength and workability can be ensured. Therefore, the total content of P and S is preferably 0.03 mass% or less. More preferably 0.02 mass% or less.
Al: 0.08% by mass or less
Al is an element added as a deoxidizer in the steel-making stage, and is usually added in an amount of 0.01 mass% or more. However, if the Al content exceeds 0.08 mass%, inclusions such as alumina increase, and the adverse effect on fatigue resistance becomes remarkable. Therefore, the Al content is 0.08 mass% or less. Preferably 0.02 to 0.07 mass%.
N: 0.010 mass% or less
N is an element that greatly deteriorates the aging resistance of steel, and is preferably as small as possible. In particular, if N exceeds 0.010 mass%, deterioration in aging resistance becomes significant, and therefore the N content is 0.010 mass% or less. The lower limit of N is not particularly limited, but is preferably about 0.001 mass% from the viewpoint of preventing an increase in production cost.
In order to further improve the strength of the steel sheet and the peel strength of the welded portion, it is preferable that at least 1 steel sheet constituting the welded joint of the present invention contains at least 1 component of the following groups a and B in addition to the above component composition.
Group A: from Ti: 0.0005 to 0.01 mass% and Nb: 0.005-0.050% by mass of 1 or 2 kinds selected from
Both Ti and Nb have the effect of forming carbides and nitrides and precipitating, thereby suppressing the coarsening of austenite during annealing in the production of steel sheet. In order to obtain the above-mentioned effects, it is preferable that 1 or 2 selected from Ti and Nb contain 0.0005 mass% or more of Ti and 0.005 mass% or more of Nb. However, even if Ti and Nb are contained excessively, the above effects are saturated, and only the raw material cost is increased. Further, since the recrystallization temperature is increased, the metal structure after annealing in the production of the steel sheet may become uneven, and the stretch-flange formability may be impaired. Further, the precipitation amount of carbide or nitride increases, and the yield ratio may increase, thereby deteriorating the shape freezing property. Thus, when Ti and/or Nb are contained, Ti is 0.01 mass% or less and Nb is 0.050 mass% or less. More preferred contents are Ti: 0.0006 to 0.0080 mass%, Nb: 0.010 to 0.040 mass%.
Group B: from Cr: 1.0 mass% or less, Mo: 0.50 mass% or less and B: 0.10% by mass or less of 1 or 2 or more
Cr, Mo, and B are effective elements for improving the hardenability of steel, and in order to obtain the above effects, Cr: 0.01 mass% or more, Mo: 0.004 mass% or more and B: 0.0001% by mass or more of 1 or more. However, even if these elements are contained in excess, the above effects are saturated, and only the raw material cost is increased. Thus, when Cr, Mo, and B are contained, it is preferable to add Cr: 1.0 mass% or less, Mo: 0.50 mass% or less, B: 0.10% by mass or less. More preferably, Cr: 0.02 to 0.50 mass%, Mo: 0.010-0.10 mass%, B: 0.001 to 0.03 mass%.
The balance of the steel sheets constituting the welded joint of the present invention other than the above components is Fe and inevitable impurities.
Tensile strength of steel sheet
Preferably, at least 1 steel sheet among the plurality of steel sheets constituting the lap laser spot welded joint of the present invention is a high tensile steel sheet having a tensile strength TS of 980MPa or more. When at least 1 steel sheet is the high-tensile steel sheet described above, the lap laser spot welded joint can obtain high joint strength, and even when a welding defect occurs in the conventional oval joint, the generation of weld cracks can be prevented because stress concentration to the final set portion is small in the oval joint portion of the present invention. Therefore, for example, it is preferable that at least 1 steel sheet among the plurality of steel sheets has the above composition and has a tensile strength TS of 980MPa or more. The plurality of steel sheets constituting the lap laser welded joint of the present invention may be steel sheets of the same composition and the same strength, or steel sheets of different compositions and different strengths.
Method for manufacturing lap laser spot welded joint
Next, a method for manufacturing a lap laser spot welded joint according to the present invention will be described with reference to fig. 4 to 6.
In the method of manufacturing a lap laser spot welded joint according to the present invention, a plurality of steel sheets are vertically overlapped and a laser beam is intermittently irradiated onto the uppermost steel sheet surface of the overlapped plurality of steel sheets to sequentially form the joint portion 4, thereby forming a welded portion and manufacturing a welded joint. In the example shown in fig. 4, in the lap laser welded joint 1 of the present invention, the laser beam 6 is intermittently irradiated onto the surface of the uppermost steel sheet 2 while a plurality of steel sheets 2 and 3 are stacked, and the joint 4 between the steel sheets 2 and 3 is continuously formed in a row shape to perform lap laser beam welding.
As described above, in the present invention, single-side welding is performed on a plurality of steel sheets that are stacked. By adopting the one-side welding, the working space required for the welding can be reduced.
In one-side welding, it is preferable that the laser beam is irradiated from the side of the steel sheet having a large thickness among the plurality of steel sheets stacked one on another, from the viewpoint of preventing burn-through during welding. On the other hand, from the viewpoint of preventing the non-joint due to the non-penetration, it is preferable to irradiate the laser beam from the steel sheet side having a small plate thickness. When the steel sheets have the same thickness, the laser beam may be irradiated from any steel sheet side.
In the present invention, it is important that the lap laser welded joint of the present invention is set such that the ratio (G/T) of the total gap G between the steel sheets constituting the welded portion to the total thickness T of the steel sheets constituting the welded portion is 0 to 0.15, that is, the ratio of the total gap G between the steel sheets constituting the welded portion to the total thickness T of the steel sheets constituting the welded portion is in the range of 0 to 15%. This is because if the ratio of G to T exceeds 15%, the depth of the crater at the welding terminal end portion becomes deeper, and stress becomes more likely to concentrate. Preferably 0 to 10%.
Further, most importantly in the present invention, as shown in fig. 5 (a), the laser beam 6 is spirally scanned from the outer side to the inner side of the oblong shape while being spun so as to draw an elongated circle in which a semicircle and a straight line are combined, thereby forming an oblong joint portion larger than usual. As described above, since the laser beam is welded while being spun, the occurrence of spattering can be suppressed, and therefore, as shown in fig. 3 (b), the thickness u of the central portion 4a, which is the final solidification portion of the joint, can be increased, and excessive stress concentration acting thereon can be prevented. Therefore, the generation of cracks can be prevented.
Here, as shown in fig. 5 (a) and (b), the size of the oblong joint can be changed by adjusting the spin radius R when the laser beam is spun, the amount of progress c, which is the distance by which the laser beam advances for every 1 revolution, and the length L of the oblong linear portion and the radius R of the arcuate portion drawn by the laser beam. The spin radius R, the amount of progress c, the length L of the oblong straight portion, and the radius R of the arcuate portion are adjusted so that the minor axis width D of the oblong joint portion is equal to or smaller than the minor axis width L of the oblong joint portion1Major axis width D2And the minimum thickness u of the final solidified part satisfies the following formulae (2) to (5):
2.0≤D1≤8.0 (2)
6.0≤D2≤15.0 (3)
1.1≤D2/D1≤5.0 (4)
0.6≤u/T≤1.0 (5)。
here, as the type of the laser beam used for the laser beam welding according to the present invention, for example, a fiber laser, a disk laser, or the like can be used. In order to satisfy the above (2) to (5), the laser beam is preferably irradiated at a power: 1.0 ~ 6.0kW, focus position: a beam diameter of-5 mm to +5mm from the surface of the steel plate irradiated with the laser beam: 0.2-0.6 mm and scanning speed of laser beam: within a range of 5.0 to 10.0 m/min. More preferably, the laser power: 3.0 ~ 5.0kW, focus position: steel plate surface-steel plate surface +5mm irradiated with laser beam, beam diameter: 0.3-0.5 mm and scanning speed of laser beam: 6.0-9.0 m/min.
In the above description, the case where the shape of the joint portion constituting the welded portion of the present invention is an oval shape was described, but the shape may be an oval shape as long as the above-described expressions (2) to (5) are satisfied.
Structural component for automobile body
Next, the structural member for an automobile body of the present invention will be described.
Examples of lap laser spot welded joints to which the present invention can be preferably applied include structural members (strength members) that form a framework portion of an automobile body. The member shown in fig. 1 is formed of a steel plate 2 as a frame member and a steel plate 3 as a panel member, each having a substantially hat-shaped cross section, and a flange portion 2b of the steel plate 2 and the steel plate 3 disposed to face the flange portion 2b are joined to each other by a welded portion including a row-shaped continuous oval joint portion 4 formed by the laser beam welding. In order to apply a member having such a shape to a strength member of an automobile body, it is important that the strength of a welded portion is excellent from the viewpoint of ensuring collision safety, and the lap laser spot weld joint of the present invention has sufficient peel strength without cracking at a final solidification portion of a joint portion, and therefore can be applied to, for example, a structural member such as a center pillar or a roof rail of an automobile body.
Here, a preferred position for forming a weld portion when manufacturing a structural member for an automobile body or the like by applying the lap laser welded joint of the present invention will be described by taking, as an example, a case where 2 steel sheets 2 and 3 having L-shaped cross sections of flange portions 2b and 3b are overlapped so that the flange portions face each other and laser beam welding is performed from one side, as shown in fig. 6. Fig. 6 (a) is a plan view of the overlapped flange portions as viewed from above, showing that a welded portion including a row-like continuous oblong joint portion is formed in the flange portion, and fig. 6 (b) is a cross-sectional view of the C-C cross-section shown in the above (a).
In fig. 6, when the preferred position for forming the welded portion is defined as a welding position X by taking the center line of the thicknesses of the steel sheets 2 and 3 as a base point (point 0) and defining the distance from the base point to the widthwise central portion of the oblong joint portion formed in the flange portion, the welding position X preferably satisfies the following formula (5):
5t≤X≤8t (5)
wherein, t: the thickness (mm) of the thickest steel sheet among the steel sheets constituting the welded portion.
For example, when the thickness t of the thickest steel sheet is 2mm, the welding position X is preferably in the range of 10 to 16 mm.
This is because, if the welding position X is less than 5t, the welded metal portion may be easily broken in a peel test, and the peel strength may be reduced. On the other hand, if the welding position X is greater than 8t, the moment received by the 1 st joint portion 4 and the subsequent joint portion 5 in the peel test becomes too large, and the peel strength is still reduced. A more preferable range of X is 6 t.ltoreq.X.ltoreq.7 t. By forming the welded portion at the above-mentioned position, the peel strength of the welded joint portion where 2 sheets of steel sheet overlap each other with a total sheet thickness of 2 to 5mm can be made 12.0kN or more.
The formula (5) for the welding position X is not limited to the T-shaped lap laser beam welded joint in which 2 steel plates having an L-shaped cross section are overlapped as shown in fig. 6, and may be applied to a lap laser spot welded joint obtained by laser beam welding a frame component (steel plate 2) and a panel component (steel plate 3) having a substantially hat-shaped cross section as shown in fig. 1, for example, and the base point (0 point) of the welding position X in this case may be the plate thickness center of the vertical wall portion 2a of the frame component having a substantially hat-shaped cross section.
Examples
The width was measured from high tensile steel sheets having the composition of A to J shown in Table 1, any of the thicknesses of 1.2mm, 1.6mm and 2.0mm, and the tensile strength TS of 590 to 1180 MPa: 100mm, length: a150 mm sample was bent into an L-shape having a long side of 120mm and a short side of 30mm to produce an L-shaped steel sheet. Here, the long side of the L-shaped steel plate corresponds to the vertical wall 2a in fig. 1 (fig. 4), and the short side corresponds to the flange 2b in fig. 1 (fig. 6). Next, as shown in fig. 7, after the 2L-shaped steel plates 7 were stacked so that their short sides were opposed to each other, the stacked portions were irradiated with a laser beam in the atmosphere to form oval joint portions arranged intermittently, and a T-shaped peel test piece 8 was prepared.
In the lap laser beam welding, a laser beam having a beam diameter of 0.4mm phi at the focal position was used, and the focal position was set to the upper surface of the steel plate (the surface of the upper steel plate 7 shown in fig. 7) to be overlapped, and as shown in table 2, the above-described lap laser beam welding was performed on the steel plateThe gap G between 2 steel plates, the power P of the irradiated laser beam, the scanning speed v, the radius R of the laser beam during spinning, the amount of progress c per 1-time spinning, and the length L of the oblong linear portion and the radius R of the circular arc portion drawn by the laser beam were varied in various ways so that the minor-axis width D of the oblong joint portion was varied1Major axis width D2And the minimum thickness u of the final solidified portion were variously changed as shown in table 2. At this time, the welding position X at which the welded portion is formed is set to 6.5 times (fixed) the maximum plate thickness t.
The T-shaped peel test piece obtained as described above was subjected to visual inspection and penetrant testing to determine whether or not cracks were generated in the welded portion, particularly in the final solidified portion of the joint portion, and whether or not burn-through was present.
Then, a tensile test was performed on the T-shaped peel test piece at a speed of 10mm/min with the longitudinal direction of the long side of the 2-piece L-shaped steel sheet being the tensile direction, and the peel strength (maximum load) was measured. In the present example, the case where the peel strength was 12.0kN or more was determined as "pass".
[ Table 1]
[ Table 2-1]
[ tables 2-2]
[ tables 2 to 3]
The results of determination of the presence or absence of weld cracking and the results of measurement of peel strength are shown in table 2.
From the results, it was found that the test pieces (nos. 1, 9, 17, 25, 33, 41, 49, 57, 65 and 73) obtained by lap laser beam welding under the conditions satisfying the present invention had no cracks at the finally solidified portions of the joined portions, had no burn-through, and had a peel strength of 12.0kN or more.
In contrast, in the test pieces of nos. 2, 10, 18, 26, 34, 42, 50, 58, 66 and 74, the minimum thickness u of the final solidified portion was less than 60% of the total sheet thickness T, and therefore, although no burn-through occurred, cracks were generated in the final solidified portion of the joint portion, and the peel strength was also less than 12.0 kN.
In the test pieces of nos. 3, 11, 19, 27, 35, 43, 51, 59, 67 and 75, the gap G of the welded portion was larger than 15% of the total thickness T of the steel sheet, and therefore, although there was no burn-through, cracks were generated in the finally solidified portions of the joined portions, and the peel strength was also lower than 12.0 kN.
In addition, in the test pieces of Nos. 4, 12, 20, 28, 36, 44, 52, 60, 68 and 76, the minor axis width D of the joint portion was determined1All of which are less than 2mm, so that cracks were generated in the finally solidified portions of the joint portions without burn-through, and the peel strength was also less than 12.0 kN.
In addition, in the test pieces of Nos. 5, 13, 21, 29, 37, 45, 53, 61, 69 and 77, the minor axis width D of the joint portion was determined1All of which are larger than 8mm, so that burn-through occurs although there is no crack in the finally solidified portion of the joint.
In addition, in the test pieces of Nos. 6, 14, 22, 30, 38, 46, 54, 62, 70 and 78, the minor axis width D of the joint portion was determined2All of which are larger than 15mm, so that cracks were generated in the finally solidified portions of the joint portions, although they were not burned through.
In addition, in the test pieces of Nos. 7, 15, 23, 31, 39, 47, 55, 63, 71 and 79, the major axis width D of the joint portion was determined2Width D relative to minor axis1Ratio (D)2/D1) All of which are larger than 5.0, so that cracks were generated in the finally solidified portions of the joint portions, although there was no burn-through。
In addition, in the test pieces of Nos. 8, 16, 24, 32, 40, 48, 56, 64, 72 and 80, the major axis width D of the joint portion was determined2Width D relative to minor axis1Ratio (D)2/D1) All of which are less than 1.1, so that cracks occur in the finally solidified portion of the joint portion, although the burn-through is not yet caused.
Further, as a result of a test piece obtained by lap laser beam welding 2 steel sheets having different strength grades under the condition satisfying the present invention, nos. 81 and 82 showed a combination of 590MPa grade and 980MPa grade, but No.81 having a steel composition within the preferable range of the present invention had no weld crack and also had excellent peel strength, whereas No.82 having a steel composition outside the preferable range of the present invention had weld crack and had peel strength lower than 12.0 kN.
As described above, in all of the examples of the present invention in which the lap laser beam welding was performed according to the present invention, good lap laser beam welded joints having the characteristics desired by the present invention were obtained, whereas in the comparative examples in which the conditions of the present invention were not satisfied, good lap laser beam welded joints could not be obtained.
Industrial applicability
The technique of the present invention can realize high-speed and low-strain welding, and therefore can be applied to an automobile structural member having a flange portion.
Description of the reference numerals
1: lap laser spot welding joint
2. 3: steel plate
2 a: vertical wall part of steel plate 2
2 b: flange part of steel plate 2
4. 14: joint (solder joint)
4a, 14 a: final solidification part (center part) of the joint part
5: cracking of the final set portion of the joint
6: laser beam
7: l-shaped steel plate
7 a: long side of L-shaped steel plate
7 b: width of L-shaped steel plate
8: peeling test piece
S: laser beam irradiation start portion in forming joint portion
E: laser beam irradiation end portion for forming joint portion
σ a, σ b: stress acting on final solidification portion of joint portion
T: total thickness of steel plates constituting joint
u: minimum thickness of final set portion of joint
D1: minor axis width of oblong joint
D2: major axis width of oblong joint
G: total gap between steel plates constituting joint part
X: welding position
0: base point of welding position
Claims (6)
1. A lap joint laser spot welded joint having a welded portion formed by overlapping a plurality of steel plates, characterized in that,
the total gap G between the steel plates constituting the welded portion is in the range of 0 to 15% of the total thickness T of the steel plates constituting the welded portion,
the welding part comprises oblong joint parts arranged intermittently, and
the oblong joint portion satisfies all of the following formulas (1) to (5):
1.0≤T≤6.0 (1)，
2.0≤D1≤8.0 (2)，
6.0≤D2≤15.0 (3)，
1.1≤D2/D1≤5.0 (4)，
0.6≤u/T≤1.0 (5)，
wherein T is a total plate thickness (mm) of the steel plates constituting the welded portion,
D1is the minor axis width (mm) of the oblong joint,
D2is long and roundThe long axis width (mm) of the form-fitting part,
u is the minimum thickness (mm) of the final set portion of the oblong joint.
2. Lap laser spot weld joint according to claim 1, characterized in that the composition of at least 1 of said steel sheets contains C: 0.07 to 0.4 mass%, Si: 0.2 to 3.5 mass%, Mn: 1.8-5.5 mass%, P + S: 0.03 mass% or less, Al: 0.08% by mass or less and N: 0.010 mass% or less, and the balance of Fe and inevitable impurities.
3. Lap laser spot welded joint according to claim 2, characterized in that said steel sheet contains, in addition to said composition, at least 1 of the following groups a and B of components:
group A is selected from Ti: 0.0005 to 0.01 mass% and Nb: 0.005-0.050% by mass of 1 or 2 kinds selected from;
group B is selected from Cr: 1.0 mass% or less, Mo: 0.50 mass% or less and B: 0.10% by mass or less of 1 or 2 or more selected from the group.
4. A lap laser spot weld joint according to any one of claims 1 to 3, characterized in that at least 1 of the steel sheets is a high-tensile steel sheet having a tensile strength of 980MPa or more.
5. A method of manufacturing a lap laser spot welded joint, wherein a plurality of steel plates are vertically overlapped, a laser beam is intermittently irradiated onto one side surface of the overlapped steel plates, a welded portion including oblong joint portions continuously arranged in a row is formed to manufacture a lap laser spot welded joint,
the manufacturing method is characterized in that the manufacturing method comprises the following steps,
the total gap G between the steel plates constituting the welded part is set within the range of 0-15% of the total thickness T of the steel plates constituting the welded part,
the oval joint is formed by spirally scanning a laser beam from the outer side to the inner side of the oval while spinning the laser beam so as to draw an elongated circle in which a semicircle and a straight line are combined, and,
controlling at least 1 of laser power, focal position, welding speed, spin radius, and amount of movement per 1 spin and beam diameter such that the oblong joint satisfies all of the following equations (1) to (5):
1.0≤T≤6.0 (1)，
2.0≤D1≤8.0 (2)，
6.0≤D2≤15.0 (3)，
1.1≤D2/D1≤5.0 (4)，
0.6≤u/T≤1.0 (5)，
wherein T is a total plate thickness (mm) of the steel plates constituting the welded portion,
D1is the minor axis width (mm) of the oblong joint,
D2the long axis width (mm) of the oblong joint,
u is the minimum thickness (mm) of the final set portion of the oblong joint.
6. A structural member for an automobile body having the lap laser spot welded joint according to any one of claims 1 to 4.
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| PCT/JP2019/013704 WO2020194687A1 (en) | 2019-03-28 | 2019-03-28 | Lap laser spot weld joint and method for producing same, and automotive body structural component |
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| CN113573837A true CN113573837A (en) | 2021-10-29 |
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| JP (1) | JP7211491B2 (en) |
| KR (1) | KR102597121B1 (en) |
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| CN112404129B (en) * | 2020-10-27 | 2023-05-02 | 苏州大学 | Welding method of steel plate for hot-rolled automobile |
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| JP6443319B2 (en) * | 2015-12-24 | 2018-12-26 | Ｊｆｅスチール株式会社 | Lap laser spot welded joint and method of manufacturing the welded joint |
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2019
- 2019-03-28 CN CN201980094063.9A patent/CN113573837A/en active Pending
- 2019-03-28 JP JP2021508617A patent/JP7211491B2/en active Active
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| JP7211491B2 (en) | 2023-01-24 |
| KR20210116631A (en) | 2021-09-27 |
| JPWO2020194687A1 (en) | 2020-10-01 |
| WO2020194687A1 (en) | 2020-10-01 |
| KR102597121B1 (en) | 2023-11-01 |
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